This invention relates to a head actuator supporting a magnetic head for recording information on and/or reproducing information from a disc-shaped recording medium and for causing movement of the head in the direction along the radius of the disc-shaped recording medium for positioning the head in a target position. This invention also relates to a recording and/or reproducing apparatus incorporating this head actuator.
In a magnetic disc device or an optical disc device for recording and/or reproducing information signals for a disc-shaped recording medium, attempts are now being made to reduce the size as well as to increase the recording density thereof. For example, a hard disc device may be utilized, in which the track density and the recording density of the recording medium of the disc are raised to increase the recording capacity.
In this hard disc drive of increased recording capacity, the head needs to be precisely positioned on a target track of the disc to which the head is to be accessed.
In general, if a moving object with an inertial moment J is performing a sinusoidal movement with an amplitude A and a frequency f, the amplitude of the angular acceleration Axc2x7sin(2xcfx80ft), determined from a second order differential of the displacement with respect to time, is equal to Axc2x7(2xcfx80f)2, and the amplitude of the inertial force determined by the product of the angular acceleration and the inertial moment is equal to Jxc2x7Axc2x7(2xcfx80f)2. Therefore, the driving force T, necessary for causing movement of a moving object with an inertial moment J, is given by
Txe2x88x9dinertial moment (J)
Txe2x88x9damplitude (A)
Txe2x88x9dsquare of frequency of motion (f).
Thus, if the driving force is constant, the amount of oscillation of the moving object (amplitude) is decreased in inverse proportion to the frequency of motion.
By these mechanical properties, if, in a system in which an object having an inertial moment is kept in motion and its position is controlled, the positioning error is increased roughly in proportion to the square of the frequency, thus deteriorating the control performance.
For enlarging the servo control range of the head positioning system, it is necessary to raise the servo loop gain so that the head/track relative offset will be within the allowable residual servo error value. However, in a single-stage actuator, it has been shown that, for the following reason, the actuator driving power needs to be proportionate to the fourth power of the frequency.
That is, the relationship between the driving force T and the driving current i is given by the equation:
Ktxc2x7i=T=Jxc2x7Axc2x7(2xcfx80f)2xe2x80x83xe2x80x83(1)
where Kt is a torque constant.
On the other hand, since the driving power P is proportionate to the square of the driving current i,
Pxe2x88x9di2=((J/Kt)xc2x7Axc2x7(2xcfx80f)2)2xe2x80x83xe2x80x83(2)
so that it can be rewritten in a proportional form to
Pxe2x88x9di2xe2x88x9df4xe2x80x83xe2x80x83(3)
That is, the driving power P is proportionate to the fourth power of the frequency of motion.
Therefore, if the rotational speed (rpm) of the disc is doubled, with the head to track misregistration (track misregistration TMR) being fixed, track-related oscillations on the disc are shifted to a doubled value as a whole towards the high side. Thus, the driving power of the actuator is increased by a power of 4 from that for the original disc rotational speed, that is to a 16-fold value.
It is noted that the smaller the actuator size, the smaller the inertial moment J, such that the torque constant to inertial moment ratio Kt/J is increased. Thus, by dividing the actuator into two steps, namely the coarse movement step and the fine movement step, and by reducing the inertial moment to as small a value as is possible for the fine movement driving mechanism, the Kt/J ratio can be correspondingly increased. For example, if the single-stage actuator in current use is used as a coarse movement driving mechanism, and Kt/J of the fine driving movement mechanism is e.g., 36 times that of the coarse movement driving mechanism, the maximum frequency and the driving power that can be controlled are as follows:
That is, as for the controllable band, from equation (1) above,
(Kt/J)xc2x7i=Axe2x88x92(2xcfx80f)2.
Therefore, if the driving current i and the amplitude A are constant,
f2xe2x88x9dKt/J, so that
fxe2x88x9d{square root over ((Kt/J))}
and hence the maximum frequency f (fine) that can be controlled by the fine movement driving mechanism is
f(fine)=f(coarse)xc2x7{square root over (36)}=f(coarse)xc2x76.
Therefore, the frequency f (fine) is six times the maximum frequency f (coarse) that can be controlled by the coarse movement driving mechanism.
As for the driving power, since
Pxe2x88x9d((J/Kt)xc2x7Axc2x7(2xcfx80f)2)2
from equation (2), if the amplitude A and the frequency f are constant,
Pxe2x88x9d(J/Kt)2.
Therefore, the driving power P (fine) required for the fine movement driving mechanism is
P(fine)=P(coarse)xc2x7(1/36)2=P(coarse)xc2x7(1/1296)
or 1/1296 of the driving power P (coarse) required for the rough movement driving mechanism.
Meanwhile, the most difficult problem in improving the track follow mode is that the angular velocity of a rotary actuator in performing track following is extremely small. It has been known that the frictional force generated between the bearing and a ball shaft for an extremely small angular velocity is such that the displacement curve representing the force of rolling friction generally describes a hysteresis loop for an extremely small width of displacement of the order of the rollout angle. The hysteresis is susceptible to irregular changes dependent upon the temperature or humidity, such that, due to non-linearity of the frictional force, prediction of displacement is extremely difficult. This in turn renders positioning control difficult. Thus, in a conventional single-stage actuator, non-linearities produced by the bearing tend to frustrate attempts towards increasing the track density.
In order to cause a magnetic head of a hard disc device to follow a high rotational speed rpm and high track density associated with a conventional voice coil motor (VCM), a double-stage micro-actuator system in a variety of systems may be utilized, such as a system combining a VCM for rough movement and a piezo element for fine movement (PZT), a piggy back system employing a VCM along with rough movement/fine movement, or a system for driving a slider by a piezo element for fine movement (PZT) or an electrostatic actuator.
The micro-actuator for double stage servo is roughly classified into a type mounted on the base of a suspension for driving the head in its entirety, a type for driving a slider and a type for driving a head element.
In the two-stage servo micro-actuator, used for positioning the head to high accuracy, the following problem is met in connection with these respective types.
That is, in the type mounted on the base of the suspension for driving the entire head, it is difficult to enhance the servo bandwidth due to the mass and vibration characteristics of the suspension.
In the slider driving type, the servo range, limited by the acceleration (driving force), is narrower than that in the head element due to the mass weight of the slider.
In the head element driving type, the production process is complex and varied because of integration of the actuator formation and the head element manufacturing steps.
Therefore, insofar as the structure is concerned, the driven movable part of the actuator smaller in size and weight than the slider and can be actuated by a smaller force. In addition, the manufacturing process of the slider driving type micro-actuator is not as complex as the head element driving type and is suited to a batch process.
On the other hand, in an optical disc device, a double-stage actuator for tracking has been used. It is noted that the optical pickup is appreciably larger in size than the magnetic pickup. Since the optical pickup is not of the floating type, as in the magnetic head, the focusing thereof can be controlled electromagnetically. Usually, the optical disc device includes a single sole optical pickup. For achieving an optical pickup which is composed of plural heads and plural discs, an optical pickup as small in size and weight as a magnetic head is required.
It is therefore an object of the present invention to provide a head actuator of the slider driving type wherein the driven moving part is smaller in size and weight than the slider and can be moved with a smaller force.
It is also an object of the present invention to provide a head actuator of the slider driving type which can be manufactured by a process less complex than in the head element driving type and can be more suited to a batch process.
It is a further object of the present invention to provide a recording and/or reproducing apparatus employing this head actuator of the slider driving tape.
It is yet another object of the present invention to provide a head actuator for a floating type optical head for recording and/or reproducing which is small in size and weight and which does not require a focussing servo.
It is a still further object of the present invention to provide a recording and/or reproducing apparatus employing this head actuator for a floating type optical head.
In one aspect, the present invention provides a head actuator including a head for recording information on and/or reproducing information from a disc-shaped recording medium. The head is moved in the radial direction of the disc-shaped recording medium for positioning the head at a target position. The head actuator also includes a head slider and a movable member carrying the head slider. The head slider includes a base having a recording medium facing surface facing the disc-shaped recording medium, a head unit arranged so that the head will face the disc-shaped recording medium, a connecting unit for connecting the head unit to an air outlet side of the base, and an electromechanical transducer element forming at least a portion of the connecting unit. The movable member is movable in a direction along the radius of the disc-shaped recording medium. The electromechanical transducer element is moved in response to supplied driving signals for causing fine movement of the head relative to the disc-shaped recording medium along the tracking direction.
The head actuator according to the present invention includes, as the head, an optical recording and/or reproducing head, a proximate optical recording and/or reproducing head, or a magnetic recording and/or reproducing head.
The head actuator according to the present invention may include a mirror in the head unit for guiding a laser light to the recording and/or reproducing head or to the proximate optical recording and/or reproducing head.
The head actuator according to the present invention may further include an optical recording and/or reproducing head and a magnetic recording and/or reproducing head combined on a single head.
The base of the head actuator according to the present invention may be formed of, for example, Si, SiC or Al2O3xe2x80x94TiC.
The head unit of the head actuator according to the present invention may be formed of, for example, a transparent material having the refractive index of not less than 1.
The head unit of the head actuator according to the present invention may be formed of, for example, Si, SiC or Al2O3xe2x80x94TiC.
The head unit of the head actuator according to the present invention may be formed of, for example, a transparent material having the refractive index of not less than 1.
Also, the base and the head unit of the head actuator according to the present invention are formed with air lubricating surfaces.
In addition, in the head actuator according to the present invention, the electromechanical transducer element is, for example, a piezo element, which receives driving signals for causing stroke movement of the head relative to the disc-shaped recording medium along the tracking direction.
In another aspect, the present invention provides a recording and/or reproducing apparatus for causing movement of a head in a direction along the radius of the disc-shaped recording medium for recording information thereon and/or reproducing information therefrom, wherein the apparatus includes a head actuator and control means. The head actuator includes a head slider, which has a base which includes a recording medium facing surface facing the disc-shaped recording medium, a head unit arranged so that the head will face the disc-shaped recording medium, a connecting unit for connecting the head unit to an air outlet side of the base, and an electro-mechanical transducer element forming at least a portion of the connecting unit. This electro-mechanical transducer element is adapted for causing fine movement of the head along the tracking direction relative to the disc-shaped recording medium. The head actuator also has a movable member carrying the head slider. The control means generates driving signals supplied to the electro-mechanical transducer element provided in the head slider. The movable member is movable in a direction along the radius of the disc-shaped recording medium. The control means supplies the driving signals to the electro-mechanical transducer element to cause fine movement of the head relative to the disc-shaped recording medium for positioning at a target position.
In the recording and/or reproducing apparatus according to the present invention, the electro-mechanical transducer element is, for example, a piezo element. The control means supplies the driving signals to the piezo element to cause stroke movement of the head relative to the disc-shaped recording medium along the tracking direction.
In the recording and/or reproducing apparatus according to the present invention, an optical recording and/or reproducing head and a magnetic recording and/or reproducing head, combined as a single unit, can be provided as the head. A magnetic disc and an optical disc are selectively used to record and/or reproduce the information.
Specifically, the head unit, connected via the connecting unit to the base of the slider, can be smaller in size and weight than the slider, and can be moved with a smaller force by the electro-mechanical transducer element forming a portion of the connecting unit.
Thus, according to the present invention, in the slider driving type head actuator, the moving part that needs to be driven is smaller in size and weight than the slider and can be moved with a smaller force. Moreover, the manufacturing process is less complex than the head element driving type. According to the present invention, a head actuator is provided suited to batch processes. In addition, a recording and/or reproducing apparatus incorporating this head actuator of the slider driving type is provided. Moreover, according to the present invention, a head actuator for a floating type recording and/or reproducing head is provided which does not require focussing servo. Moreover, a recording and/or reproducing apparatus employing this head actuator of the floating type is provided.
Various other objects, advantages and features of the present invention will become readily apparent from the ensuing detailed description and the novel features will be particularly pointed out in the appended claims.